Secondary cells such as nickel-cadmium cells and nickel-hydrogen cells are recharged many times throughout their service life. The recharging operation must be carefully controlled in order to minimize the harmful effects that have been known among people skilled in the art (see, for example, "Charging Storage Cells: Life-Extending Ability", Bob Williams, Cellular Business, April, 1989, pp. 44-49). In the early stage of the secondary cell recharging technology, the recharging operation consumed a time as long as several hours. As consumer devices powered by the secondary cells became more widely used, however, it was necessary to develop a system that was capable of recharging the secondary cells not in hours but in minutes. Though it is possible to "quick charge" the secondary cells, more careful monitoring and control are necessary for the cell recharging process in order to prevent irreversible damage to the cells (see, for example, "Latest Information on Nickel-Cadmium Cells", September, 1990 Report in Brussels Seminar, Cadmium Association, November, 1990, London, U.K.).
There have heretofore been developed a variety of secondary cell recharging systems for quickly recharging secondary cells. These systems generally employ an electric circuit that monitors the voltage and/or the temperature of the cell that is being recharged and interrupts the supply of a charging current and/or changes the charging current to the cell when the temperature or the voltage thereof has reached a predetermined level. A representative prior art has been disclosed in U.S. Pat. No. 4,006,397 assigned to Catotti et al.
Furthermore, Japanese Patent Publications Nos. 23528/1987 and 23529/1987 disclose methods of recharging secondary cells such as nickel-cadmium cells wherein, during the recharging operation, attention is given to a change in the voltage waveform of the cells, a plurality of inflection points appearing in the voltage waveform are stored in advance, and the charging operation is interrupted when the plurality of the stored inflection points have occurred in a predetermined order. According to the above methods, however, changes in the voltage waveform during the charging operation must be recorded in advance for each of various kinds of the cells, and the stored content must be rewritten into a form that corresponds to the cell prior to executing the charging operation depending upon the kind of the cell that is to be recharged, requiring cumbersome operation. Depending upon the environment in which the charging operation is carried out and the hysteresis of the cell, furthermore, the waveform of the output voltage of the cell often does not exhibit the order or magnitude of waveforms as stored. It is not, therefore, able to properly carry out the charging operation or the recharging operation and, hence, the high-speed charging operation could not be executed without deteriorating the cell performance.
That is, the secondary cells and, particularly, the nickel-cadmium cells or the nickel-hydrogen cells have required a recharge time of, usually, six hours to 16 hours in the longest case. A time of one to two hours is required even by the so-called high-speed charging method that is intended to recharge the cells within relatively short periods of time.
When rechargeable cells, storage cells and batteries are being recharged for their respective purposes, the charging time is best kept as short as possible. Due to the bottlenecks such as the rise in temperature stemming from the principle of chemical reaction inside the secondary cell and the rise in the internal pressure, however, it is not possible to charge the cell within short periods of time by flowing a large current as it results in the destruction of the cell or in the degradation of the cell characteristics, i.e., output characteristics and charging characteristics.
In recent years, however, the secondary cells have become in demand in various fields of industries and, particularly, in the sites where machine tools are used, in the hospitals where medical appliances are used and in the communication business inclusive of mobile telephones. In these fields of use, the power source must not fail during use, and it a secondary cell that can be recharged at high speeds or, desirably, that can be recharged instantaneously, is required.
The object of the present invention, therefore, is to easily recharge a secondary cell and, particularly, the nickel-cadmium cell or the nickel-hydrogen cell within a period of time which is as short as about several minutes and not longer than 20 minutes by improving the above-mentioned defects inherent in the prior art. The recharging at this very high rate amplifies the importance of some parameters that were not so significant in the relatively low speed recharging system of the prior art. It was, however, learned that these parameters can be effectively to realize a safe and quick recharging system without harming the cells.
In executing the charging operation for the secondary cells, however, it is important to know in advance the degree of discharge of the cell and the present residual capacity. If the charging operation is carried out without knowing the residual capacity, then, the overcharging results causing the performance of the secondary cell to be deteriorated or the life to be shortened.
Such problems become serious when the above-mentioned high-speed charging method is employed.
It has therefore been desired to provide technology for correctly measuring the present residual capacity of the secondary cell that needs be charged. At present, however, no desirable method has yet been proposed for correctly measuring the residual capacity.
According to a method of measuring the residual capacity of the secondary cell that is practically employed at the present moment, for example, the cell is discharged and the output current is measured to estimate the residual capacity of the cell. This method, however, is greatly affected by the output voltage, temperature and the kind of the cell, and fails to serve as reliable means for correctly measuring the residual capacity of the cell.
The object of the present invention is to provide a device for indicating the residual capacity of the secondary cells which is capable of measuring the residual capacity of the secondary cell such as a nickel-cadmium cell or a nickel-hydrogen cell that needs be recharged correctly and within short periods of time relying upon a simple constitution by improving the aforementioned defects inherent in the prior art, and a method of indicating the residual capacity of the secondary cells.